It is known that the running surfaces of cylinders of internal combustion engines, pumps and the like are coated with layers of materials particularly resistant to corrosion, wear, etc. These coatings are typically obtained through galvanic deposition with apparatuses known in the field, such as for example those disclosed in U.S. Pat. No. 5,552,026 or U.S. Pat. No. 5,645,641.
In brief, in said apparatuses there is used an electrolyte fed inside the cylinders through piping and drained from the cylinders through a central duct that acts as anode, the cathode being connected to the cylinder block. In order to obtain a proper coating on the whole height of the cylinder, the anode usually reaches the top of the cylinder or close thereto, so that the electrolyte fills the cylinder prior to flowing out through the hollow anode.
This type of apparatus works properly, although with some drawbacks, for open cylinders, i.e. those with the head separate from the cylinder body. On the contrary, a significant difficulty arises in the case of closed cylinders, i.e. those with the head integral with the cylinder body, since it would be necessary to decrease the electrical field at the top of the cylinder. This is a problem in that merely decreasing the intensity of the current circulating between anode and cathode would imply a decrease in the electrical field along the whole height of the cylinder, which would result in an insufficient deposition on the rest of the internal surface of the cylinder.
Said decrease at the top is necessary to be able to obtain a proper coating up to the corner between the side wall and the head of the cylinder, and also to minimize the deposition of the coating on the vault of the combustion chamber formed in the cylinder head.
In fact, due to the Faraday cage effects caused by the geometry of the top of a closed cylinder, the same electrical field that in the rest of the cylinder guarantees a good deposition, at the corners turns out to be too strong for a proper deposition. Moreover, the deposition on the vault of the combustion chamber is to be avoided because said surface, in use, reaches temperatures so high as to damage the coating that can break off and fall into the cylinder causing damage.
In the case of open cylinders, that are generally flared at the top, the intense electrical field causes an excessive build-up of coating material at the sharp edges of the flaring. As a consequence, the cylinder may require an additional working to remove said build-up, or a previous working to radius the flaring in order to eliminate the sharp edges.
Therefore the object of the present invention is to provide an anode which overcomes the above-mentioned drawbacks. This object is achieved by means of an anode whose top portion has a plurality of passages that imply a reduction in surface by at least 50%, which results in a corresponding decrease in the electrical field. Other advantageous features of the present anode are disclosed in the dependent claims.